This invention relates generally to exhaust scrubbers for internal combustion engines, and more particularly to water scrubbers of the type which aspirate water into the stream of hot exhaust gases to cool them and remove pollutants before exhausting the gases into the atmosphere.
Reducing the pollutants and high temperatures of internal combustion engine exhaust and the temperatures of hot surfaces in underground mining vehicles is essential to the safety of mine personnel in underground coal mines. Diesel engines exhaust hot sulfurous gases, aldehydes, nitrogen oxide, unburned hydrocarbons and particulate pollutants. Hot exhaust emissions, typically 800.degree. to 1000.degree. F., and hot engine surfaces can ignite combustible gases and material present in underground mines, such as methane and coal dust.
Water jacketing the engine's exhaust manifold and piping in conjunction with the engine's cooling system reduces engine surface temperatures sufficiently to meet safety requirements. However, the safety hazard and pollution problems caused by hot exhaust emissions have proven more difficult to solve. Large volumes of ventilation air must be pumped into deep mines at great cost to dilute the exhaust gases of each diesel engine used in the mines and ventilation alone is insufficient to allay the safety hazards of hot exhaust gases in mines. Therefore, a variety of devices have been proposed for cooling and removing pollutants from the exhaust gases of internal combustion engines used in underground mines.
One type of device, of which the exhaust cleaner proposed by White in U.S. Pat. No. 3,383,854 is an example, sprays water or foam directly into the exhaust gas stream as it flows along a tortuous path. However, such devices increase engine back pressure, reducing effective horsepower and time between overhauls. Water mixed with the exhaust gases becomes highly acidic and dirty with exhaust particulate matter. Consequently, the spray-type devices are also unreliable because the dirty, corrosive water fouls the pumps, nozzles and filters used in such devices, necessitating frequent cleaning. In addition, such parts must be made of stainless steel, which is very expensive.
A second type of device bubbles exhaust gases through a water bath and then passes the gases through a water eliminator, as disclosed in U.S. Pat. No. 3,768,981 to Alliger and U.S. Pat. No. 3,561,194 to Baldwin, et al. Water bath devices increase back pressure on the engine more than is acceptable, particularly when full. Also, the pump used in the Baldwin device is likely to be fouled.
In a third kind of device, a stream of exhaust gases passes through a venturi tube to suck water droplets from a water chamber into the gas stream, as disclosed in U.S. Pat. No. 4,137,715 to Tung-Lung et al. However, such devices tend to be very sensitive to water levels. Filters, when used in such devices, also require frequent cleaning.
Besides the foregoing problems, many such devices inadequately mix the water and gases to effectively cleanse the gases. Also, many such devices discharge substantial amounts of entrained water together with the gases, resulting in excessive water consumption and steam generation which strains the water storage capacity of the devices and impairs mine visibility.
Because of the inadequacies of the foregoing devices, aspirated or induced water scrubbers, such as that disclosed in U.S. Pat. No. 3,976,456 to Alcock, have come into wide use. Such scrubbers typically include a scrubber tank partially filled with water or other scrubbing liquid. The exhaust gases enter the tank above the water level, are piped downwardly below the water level, and then directed upwardly at an angle. A submerged slot or orifice in the region where the gases change direction admits water into the gas stream. The gas-water mixture then enters an expansion and mixing chamber and flows upwardly above the water level. At the top of the mixing chamber the mixture is redirected downwardly by a curved duct and discharged over the water, impelling the water droplets and particles in the mixture into the water. The cooled exhaust gases are then discharged into the atmosphere.
Such scrubbers are of two types. A make-up type scrubber has a make-up water tank or reservoir and a make-up float valve for adding water to the scrubber tank to maintain a desired water level. A batch-type scrubber has a large tank for containing enough water to run for an extended period of time before refilling. Both types are provided with a float-operated safety valve for shutting down the engine if the water level drops below a safe minimum.
Engine back pressure is lower with aspirated water scrubbers than with other exhaust cleaning or cooling devices, but a number of other problems remain unsolved. For example, aspirated water scrubbers similar in principle to the Alcock device, have a specific water consumption of about 0.25 to 0.3 gallons per brake horsepower hour. However, an even lower water consumption is desired to further reduce water storage requirements and steam generation.
At the same time, existing scrubbers only remove about 30% of particulate exhaust emissions. About 90% of the particles emitted are less than 0.5 microns in diameter. Existing scrubbers either do not aspirate enough water or do not mix the water that is aspirated well enough with the gases to wet and thereby trap such small particles. In the Alcock device, water is not aspirated uniformly into the flowing gases. Moreover, the curvature of the expansion tube inhibits mixing of the water with the gases. The water is aspirated into the lower portion of the gas stream and most of it follows the outside curves of expansion tube under the influence of centrifugal force.
Another problem is that the water in such scrubbers sloshes as the vehicle moves and the water level in the tank shifts in response to changes in grade. The submerged orifice of such scrubbers can thus be briefly subjected to unacceptably low water levels or even exposed so that cooled, cleansed exhaust gases rather than water are drawn into the stream of hot, dirty gases, causing a momentary reduction or lapse in the scrubbing action. Changes in water level due to sloshing or grade changes can also actuate the safety valve, causing premature shutdown of the engine. Attempts to solve this problem by use of baffles and a separate float chamber have not been successful.
Yet another problem with existing aspirator-type scrubbers is the difficulty of producing such scrubbers and their concomitantly high manufacturing cost. In particular, the convergent inlet section and curved divergent expansion chamber are quite expensive to manufacture.
A further problem is the poor reliability of float-type water valves when they are exposed to the dirty, highly corrosive scrubbing liquid inside the scrubber tank. They frequently malfunction or may fail to shut down the engine when the water level gets too low. Use of stainless steel, rather than brass floats and valves adds much to the cost of make-up type scrubbers. This problem has led mine equipment users to preferentially use batch-type aspirated water scrubbers and to use thermally-actuated engine shutdown control valves which sense the temperature of discharged exhaust gases. However, existing batch-type scrubbers pose additional problems.
Because they contain all the water required for up to 8 hours of operation, batch-type scrubber tanks have an initially higher water level than make-up scrubbers. In existing batch scrubber designs, the higher level causes excessive engine back pressure and discharge of water. Such scrubbers must also operate over a wide range of water levels, but existing scrubbers have not done so successfully. For example, the Alcock design has a water level range of only about 5 inches, which would require a very large tank to hold sufficient water to run 8 hours if used as a batch-type scrubber. Such a large tank requires more extensive baffling and stiffening to minimize sloshing and withstand the high pressures of explosions than a smaller tank would require.
Use of thermally-actuated control valves does not insure engine shutdown if the water level becomes too low. Thus, the scrubber can fail to effectively arrest flame propagation in the event of an explosion. This possibility necessitates a flame arrester in the exhaust gas orifice. However, flame arresters add to the expense of the scrubber, increase engine back pressure, particularly when they become dirty, and require daily cleaning.
Accordingly, there is a need for an improved exhaust gas scrubber of the aspirator type for internal combustion engines, especially such engines of underground mining vehicles, for overcoming the foregoing problems.